A magnetic memory, especially a magnetic random access memory (MRAM) is a nonvolatile memory capable of a high-speed operation and rewriting of an infinite number of times. Therefore, some types of MRAMs have been put into practical use, and some types of MRAMs have been developing to improve their general versatility. In the MRAM, magnetic material is used for a memory element, and data is stored in the memory element as a magnetization direction of the memory element. To write desired data to the memory element, the magnetization direction of the magnetic material is made to be switched to a direction corresponding to the data. Some methods for switching the magnetization direction of the memory element are proposed. Those methods have in common with usage of a current (hereinafter referred to as writing current. To put a MRAM into practical use, it is important to reduce the writing current as much as possible.
According to the non-patent literature 1 (N. Sakimura et al., “MRAM Cell Technology for Over 500-MHz SoC”, IEEE Journal of Solid-State Circuits, vol. 42, p. 830-838, 2007), by reducing the wiring current equal to or less than 0.5 mA, its cell area can be equivalent to that of the existing embedded SRAM.
The most general method of writing data in a MRAM is to switch a magnetization direction of a magnetic memory element by a magnetic field which is generated by passing current through a wiring line for a writing operation prepared on the periphery of the magnetic memory element. According to this method, since the MRAM can theoretically perform writing at a speed of 1 nano-second or less, the MRAM is suitable for a high-speed MRAM. However, a magnetic field for switching magnetization of magnetic material securing thermal stability and resistance against external disturbance magnetic field is generally a few dozens of [Oe]. In order to generate such magnetic field, a writing current of about a few mA is needed. In this case, a chip area is necessarily large and power consumed for writing increases. Therefore, this MRAM is not competitive with other kinds of random access memories. In addition, when a size of a memory cell is miniaturized, a writing current further increases and is not scaling, which is not preferable.
Recently, as methods to solving these problems, following two methods are proposed.
The first method is a spin torque transfer method. According to the s spin torque transfer method, in a lamination layer including a first magnetic layer which has magnetization that can be switched, and a second magnetic layer which is electrically connected to the first magnetic layer and has magnetization that is fixed, writing current flows between the second magnetic layer and the first magnetic layer. Here, by using an interaction between spin-polarized conduction electrons and localized electrons in the first magnetic layer, the magnetization in the first magnetic layer can be switched. A reading operation is carried out by using a magnetoresistive effect generated between the first magnetic layer and the second magnetic layer. Therefore, the magnetic memory element using the spin torque transfer is an element having two terminals. The spin torque transfer is generated when a current density is equal to or more than a certain value. Accordingly, as the size of the element decreases, the writing current is also reduced. In other words, the spin torque transfer is excellent in scaling performance. However, generally, an insulating film is provided between the first magnetic layer and the second magnetic layer and relatively large current should be made to flow through the insulating film in the writing operation. Thus, there are problems regarding resistance to rewriting and reliability. In addition, there is concern that a writing error occurs in the reading operation because a current path of the writing operation is the same as that of the reading operation. As mentioned above, although the spin torque transfer is excellent in scaling performance, there are some obstacles to put it into practical use.
The second method is a current driven domain wall motion method. A MRAM using the current driven domain wall motion is disclosed in the patent literature 1 (Japanese patent publication JP2005-191032A). In a general MRAM using the current driven domain wall motion, a magnetic layer (a domain wall motion layer in which data is stored) having magnetization which can be switched is provided, and magnetization of both end portions of the domain wall motion layer is fixed such that the magnetization of one end portion is approximately anti-parallel to that of the other end portion. By providing such a magnetization arrangement, a domain wall is introduced into the domain wall motion layer. Here, as reported in the non-patent literature 2 (A. Yamaguchi et al., “Real-Space Observation of Current-Driven Domain Wall Motion in Submicron Magnetic Wires”, Physical Review Letters, vol. 92, p. 077205, 2004), when current flows in a direction passing through the domain wall, the domain wall moves in a direction which is the same as that of the conduction electrons. Therefore, by making writing current flow in an in-plane direction in the domain wall motion layer, the domain wall is made to move in a direction corresponding to the current direction, thereby enable to write desired data in the domain wall motion layer. When data is read, by using a magnetic tunnel junction including a region where the domain wall moves, the data reading is performed based on the magnetoresistive effect. Therefore, the magnetic memory element using the current driven domain wall motion method is an element having three terminals. Similar to the spin torque transfer, the current driven domain wall motion arises, when the current density is equal to or more than a certain value. Thus, the current driven domain wall motion method is also excellent in scaling performance. In addition, in the current driven domain wall motion method, the writing current does not flow through the insulating layer and the current path of the writing operation is different from that of the reading operation. Consequently, the above-mentioned problems caused in the spin torque transfer can be solved.
Meanwhile, the non-patent literature 2 discloses that a current density of approximately 1×108 A/cm2 is required for the current driven domain wall motion.
The non-patent literature 3 (S. Fukami et al., “Micromagnetic analysis of current driven domain wall motion in nanostrips with perpendicular magnetic anisotropy”, Journal of Applied Physics, vol. 103, p. 07E718, 2008) discloses usefulness of magnetic material having perpendicular magnetic anisotropy in the current driven domain wall motion method. Specifically, according to the micromagnetic simulation, if the domain wall motion layer where the domain wall motion arises has perpendicular magnetic anisotropy, the writing current can be sufficiently reduced.
The patent literature 3 (International Publication WO/2009/001706) discloses that a magnetoresistive effect element using magnetic material with perpendicular magnetic anisotropy and a MRAM using it for a memory cell. FIG. 1 is a sectional view schematically showing a magnetoresistive effect element in the International Publication WO/2009/001706. The magnetoresistive effect element 170 includes a domain wall motion layer 110, a spacer layer 120 and a reference layer 130.
The domain wall motion layer 110 is composed of ferromagnetic material with perpendicular magnetic anisotropy. The domain wall motion layer 110 includes a first magnetization fixed region 111a, a second magnetization fixed region 111b and a magnetization free region 113. The magnetization fixed regions 111a, 111b are arranged on the both sides of the magnetization free region 113. The magnetization directions of the magnetization fixed regions 111a, 111b are fixed opposite (anti-parallel) to each other. For example, as shown in FIG. 1, the magnetization direction of the first magnetization fixed region 111a is fixed in the +z direction, and the magnetization direction of the second magnetization fixed region 111b is fixed in the −z direction. Meanwhile, the magnetization direction of the magnetization free region 113 can be reversed by writing current flowing from one of the magnetization fixed regions 111a, 111b to the other and becomes the +z direction or the −z direction. Therefore, based on the magnetization direction of the magnetization free region 113, a domain wall 112a or a domain wall 112b is formed inside the domain wall motion layer 110. Data is stored as the magnetization direction of the magnetization free region 113. It can be seen that data is stored as the position of the domain wall 112 (112a or 112b). The reference layer 130 which is made of ferromagnetic material and whose magnetization direction is fixed, the spacer layer 120 which is a non-magnetic layer (an insulating layer) and the magnetization free region 113 constitute a magnetic tunnel junction (MTJ). Data can be read as the magnitude of a resistance value of the MTJ
The patent literature 3 discloses that writing current can be reduced if the domain wall motion layer 110 has perpendicular magnetic anisotropy.